As electronic devices such as mobile phones or digital cameras have high functionality and small sizes, higher functions and higher integration are in demand for semiconductor devices provided in the electronic devices. Accordingly, recently, a semiconductor element such as a integrated circuit chip being directly mounted on a wiring board has been suggested so that the mounting area is made small and efficient use is achieved.
More specifically, in the semiconductor device, a semiconductor integrated circuit element (hereinafter “semiconductor element”) is mounted on a wiring board by using convex (projection) outside connection terminals called wire bumps. Insulating resin such as glass epoxy resin is used for a base part of the wiring board. Conductive layers made of copper (Cu) or the like are selectively provided on a main surface of the wiring board. The convex (projection) outside connection terminals provided on a main surface of the semiconductor element are connected to the conductive layers of the wiring board. Outside connection terminals such as spherical electrode terminals are provided on surfaces of electrodes selectively formed on another main surface of the wiring board. In the above-mentioned semiconductor device, the semiconductor element is mounted on the wiring board in a so-called flip-chip (face-down) state. The above-mentioned flip-chip mounting structure is formed by the following methods.
In a first method, when a semiconductor element is mounted on a wiring board, an underfill material such as a thermosetting adhesive is supplied on a main surface of the wiring board in advance. While the semiconductor element is mounted on the wiring board via the underfill by applying high loads so that the underfill material is spread, the underfill material is made to flow across in the entire surface area of the semiconductor element by capillary action. At the same time, the underfill material is cured by heat applied at the time when the semiconductor element is mounted.
In the first method, since the high loads are applied to the semiconductor at the same time that the underfill material is cured, the convex shaped outside connection terminal of the semiconductor element and the electrode of the wiring board are crushed against each other so as to be connected to each other.
In this pressure connecting type first method, connection of the convex shaped outside connection terminal of the semiconductor element and the electrode of the wiring board is maintained by using a contractive force for curing the underfill material by heat and a repulsive force when the convex shaped outside connection terminal of the semiconductor element is crushed, so as to make electric connection between the convex shaped outside connection terminal of the semiconductor element and the electrode of the wiring board.
In a second method, a conductive adhesive made of, for example, silver (Ag) paste is transferred to the head end of the convex shaped outside connection terminal. The convex shaped outside connection terminal of the semiconductor device and the electrode of the wiring board are connected to each other via the conductive adhesive. Then, an underfill material is applied in the vicinity of the external circumference of the semiconductor element on the wiring board. The underfill material is made to flow across the entire surface area of the semiconductor element by capillary action and is cured by heating.
It is general practice to use, as the above-mentioned conductive adhesive, silver (Ag) or a mixture or an alloy containing silver (Ag) as a main ingredient. Silver (Ag) has a low volume resistivity and low contact resistance with gold (Au) forming the convex shaped outside connection terminal of the semiconductor element.
Japanese Patent No. 3409957 describes an example where two kinds of conductive adhesives are used for a connecting part of an electrode of a wiring board and an electrode of a semiconductor element. More specifically, a structure of a projection electrode of an IC chip mounted on the wiring board in a face-down manner is suggested in Japanese Patent No. 3409957. A first conductive adhesive made of a conductive filler of silver palladium (AgPd) is transferred to the projection electrode and a second conductive adhesive made of a conductive filler of silver (Ag) is transferred to the outside of the first conductive adhesive. A thin semiconductor device having good electric properties can be easily formed by flip chip mounting using such a method.
However, as the semiconductor device has high level functions, the pitch of the convex shaped outside connection terminals becomes narrow so that the size of the convex shaped outside connection terminals become small.
In this pressure connecting type first method, high loads are applied to the semiconductor element at the same time when the underfill material is cured so that the convex shaped outside connection terminal of the semiconductor element is crushed. However, since the semiconductor element is more solid than the wiring board, a curve whose top part is situated substantially in the center of the wiring board may be formed in the wiring board where the semiconductor element is flip-chip mounted. As a result of this, a gap in a vertical direction between the semiconductor element and the wiring board becomes narrow in an area other than an area where electric connection of the semiconductor element and the wiring board is formed by the electrode and the convex shaped outside connection terminal becomes narrow.
Because of this, a particle generated during a manufacturing process of the semiconductor device enters in the underfill material positioned in the gap in the vertical direction between the semiconductor element and the wiring board so that a circuit of the semiconductor element may be broken.
If the load applied when the semiconductor element and the wiring board are connected to each other so that the gap in the vertical direction between the semiconductor element and the wiring board is made wide, a force for connecting the convex shaped outside connection terminal of the semiconductor device and the electrode of the wiring board may be degraded so that shorts between the semiconductor element and the wiring board may be generated. In other words, if the pressure connecting type first method is applied to flip chip mounting of the semiconductor element where the convex shaped outside connection terminals are formed with a narrow pitch on the wiring board, a connection limit may be generated where the connection between the semiconductor element and the wiring board is released.
In addition, in a case where a test having a heat history such as a reflow test or a temperature cycle test is performed, due to stress caused by thermal expansion based on differences of thermal expansion coefficients of the semiconductor elements, the wiring board, the underfill material, the conductive adhesive, and others, the connection of the semiconductor element and the wiring board may become loose and a short between the semiconductor element and the wiring board may be generated.
In the above-discussed second method, the semiconductor element, where the conductive adhesive of silver (Ag) paste is transferred on the head end of the convex shaped outside connection terminal, is mounted on the wiring board so that the convex shaped outside connection terminal of the semiconductor element and the electrode of the wiring board are connected to each other. Accordingly, a connection part where the convex shaped outside connection terminal of the semiconductor element and the electrode of the wiring board are connected to each other is a low elasticity body.
Accordingly, in a case where the test having a heat history such as a reflow test or a temperature cycle test is performed, although the connection between the semiconductor element and the wing board is not loosened, ion migration, where silver (Ag) contained in the conductive adhesive is ionized due to an electric field or environment and eluted in the periphery, may be easily generated. In particular, if the pitch of the convex shaped outside connection terminals becomes narrow and a large amount of silver (Ag) exists in the connection part where the convex shaped outside connection terminals of the semiconductor element and the electrodes of the wiring board are connected to each other, the shorts due to the ion migration between the semiconductor element and the wiring board may be easily generated.
Furthermore, in the example discussed in Japanese Patent No. 3409957, since the second conductive adhesive made of silver (Ag) conductive filler is formed on the outermost circumferential part of the projection electrodes, it is difficult to avoid generation of the above-mentioned silver ion migration.